Yes, there is such a drug or medication named Xanax which belongs to the benzodiazepines group of drugs. This drug is used treat panic disorders and anxiety caused by depression.
However, the 6mg might be too large a dose for your mother-in-law to take. I have read the recommended dosage of Xanax in one of the health forum. If an adult have an anxiety disorder, the recommended Xanax dose is PO 0.25-0.5 mg tid not to exceed 4mg/day in divided doses. For adults who have panic disorders, it would be PO 0.5 mg tid, max 10mg/day. 6 mg of Xanax would be a very big dose for treating a mild to moderate anxiety attacks. Also, the duration of the medication being taken daily is typically up to 4 months. Beyond that it is no longer advisable to take the pill daily.
Another important thing that one should know before taking Xanax is that it should not be used if one has allergic reactions to other benzodiazepines drugs (Librium, Tranxene, Valium, Ativan, Serax). One must also tell the doctor if they have glaucoma, kidney or liver disease, breathing problems, drug or alcohol addiction, and history of depression or suicidal thoughts because your doctor may need to adjust the dosage. It should only be taken by persons who were prescribed with because it can be a habit-forming drug.